FiiO A3 Portable Headphone Amplifier (Silver) Suitable for headphones ranging from 16 to 150 Ohms. Gain switch for matching output to headphone power needs. Superior sound quality through expertly selected components and design, utilizing OPA1642 preamp and AD8397 amp section. Fully discrete bass boost circuit increases output by 3.5dB at 60Hz, with bypass when off. Handsome all aluminum body and chassis is sturdy and shielding against EM interference. 1400mAh internal battery powers for up to 16 hours of use.